Non-volatile memories are important elements of integrated circuits due to their ability to retain data absent a power supply. Phase change materials have been investigated for use in non-volatile memory cells, including chalcogenide alloys, which are capable of stably transitioning between amorphous and crystalline phases. Each phase exhibits a particular resistance state and the resistance states distinguish the logic values of the memory element. Specifically, an amorphous state exhibits a relatively high resistance, and a crystalline state exhibits a relatively low resistance.
A conventional phase change memory element 1, illustrated in FIGS. 1A and 1B, has a layer of phase change material 8 between first and second electrodes 2, 4, which are supported by a dielectric material 6. The phase change material 8 is set to a particular resistance state according to the amount of current applied through the first and second electrodes 2, 4. To obtain an amorphous state (FIG. 1B), a relatively high write current pulse (a reset pulse) is applied through the conventional phase change memory element 1 to melt at least a portion 8a of the phase change material 8 covering the first electrode 2 for a first period of time. The current is removed and the phase change material 8 cools rapidly to a temperature below the crystallization temperature, which results in the portion 8a of the phase change material 8 covering the first electrode 2 having the amorphous state. To obtain a crystalline state (FIG. 1A), a lower current write pulse (a set pulse) is applied to the conventional phase change memory element 1 for a second period of time (typically longer in duration than the crystallization time of the amorphous phase change material) to heat the amorphous portion of the phase change material 8 to a temperature below its melting point, but above its crystallization temperature. This causes the amorphous portion 8a (FIG. 1B) of the phase change material 8 to re-crystallize to the crystalline state that is maintained once the current is removed and the phase change memory element 1 is cooled. The phase change memory element 1 is read by applying to the electrodes a read voltage, which does not change the phase state of the phase change material 8.
A sought after characteristic of non-volatile memory is low power consumption. Oftentimes, however, conventional phase change memory elements require large operating currents. As the phase change memory is scaled down to allow large scale device integration and per-bit current reduction, the programmable volume (e.g., portion 8a of FIGS. 1A and 1B) of the phase change cell is shrunk further with an increasing surface-to-volume ratio. The increased surface-to-volume ratio of the programmable volume (e.g., portion 8a of FIGS. 1A and 1B) causes increased thermal dissipation through the surface, and larger power density is required to achieve the same local heating in the programmable volume (e.g., portion 8a of FIGS. 1A and 1B). Consequently a larger current density is necessary (approximately 1×1012 Amps/m2) for the write operation of the phase change cell. The larger current density creates severe critical reliability issues such as electromigration of the phase change material atoms (e.g., germanium-antimony-tellurium (GeSbTe)) that are placed under a high electric field for the conventional phase change memory methodology in which the cell itself acts as the heating element.
It is therefore desirable to provide phase change memory elements with reduced current requirements. For phase change memory elements, it is necessary to have a current density that will heat the phase change material past its melting point and quench it in an amorphous state. One way to increase current density is to decrease the size of the first electrode (first electrode 2 of FIGS. 1A and 1B). These methods maximize the current density at the interface between the first electrode 2 and the phase change material 8. Although these conventional solutions are typically successful, it is desirable to further reduce the overall current flow in the phase change memory element, thereby reducing power consumption in certain applications, and possibly to reduce the current density passing through the phase change material to improve its reliability.
Another desired property of phase change memory is its switching reliability and consistency. Conventional phase change memory elements (e.g., phase change memory element 1 of FIGS. 1A and 1B) have phase change material layers (e.g., phase change material layer 8 of FIGS. 1A and 1B) that are not confined, and have the freedom to extend sideways. Accordingly, the interface between the migrated amorphous portions and crystalline portions of the phase change material may cause reliability issues during programming and reprogramming of the phase change cell.